Ink-jet printing methods are that eject ink in the form of fine drops for being deposited on the surface of a printing medium such as a printing paper so as to perform printing. Among such methods, especially, Japanese Patent Application Publication No. 61-059911 (1986), Japanese Patent Application Publication No. 61-059912 (1986)and Japanese Patent Application Publication No.61-059914 (1986) respectively propose a method designed so that the electro-thermal conversion element is used as an ejection energy generating element so that heat energy generated from the electro-thermal conversion element is applied to the ink to generate a bubble in the ink and to eject an ink droplet. These methods enable a high-density multiple-orifice printing head to be made available easily and thereby enable a high-resolution and a high-quality image to be printed quickly.
However, ink used in conventional ink-jet printing methods, including those described in the above-mentioned documents, contains water as a main component and a water soluble solvent having a high melting point such as the glycol for preventing the ink from drying and clogging. When such an ink is used for printing on a plain paper, an image having an adequate optical density may not be obtained owing to permeation of the ink into inside the paper and an unevenness of the optical density of the image may occur owing to probable uneven distribution of a loading filler and a sizing in the surface layer of the paper. Further, especially when printing a color image, a plurality of colors of inks are sequentially applied on the ink, which has been applied and not yet fixed, and then the applied inks may spread at a boundary portion between different colors of the image to mix together(hereinafter referred to as bleeding). This mixing of deferent colors of inks results in deterioration of a print quality.
On the other hand, there are known methods for increasing the optical density of the image or decreasing the bleeding, which apply a liquid for making the coloring materials such as a dye or a pigment insoluble (referred to as a reacting liquid in the present specification) prior to applying ink. For example, Japanese Patent Application Laid-open No. 5-202328 (1993) proposes a method for preventing the bleeding by using the reaction between the polyvalent metal ion and the carboxyl group; further, Japanese Patent Application Laid-open No. 9-207424 (1997) proposes a method for reducing the bleeding by means of the reaction among the pigment, resin emulsion and polyvalent metallic salt.
Further, there are some proposals for the method for using the reacting liquid and the ink and carrying out the efficient printing by sequentially applying the reacting liquid and the ink. For instance, Japanese Patent Application Laid-open No. 7-195823 (1995) describes a method in which printing is performed by ejecting the reacting liquid and the ink in this order, during a single scan (hereinafter may be also referred to as 1 pass). Besides, there is another known method wherein, for speeding up printing, the above described printing during 1-pass is performed during each of bidirectional two scans with the printing head (hereinafter also referred to as a bidirectional printing). Further, as illustrated in FIG. 6, the 1-pass and bidirectional printing is commonly performed so that printing for a single scanning area is completed during the single scan with the printing head, and this printing during the 1-pass is made to take place during each of the forward scan and the backward scan with the printing head. Then, a printing medium is fed by an amount corresponding to the width of the scanning area (i.e., the width of printing by the printing head) between any one scan and another scan. In FIG. 6, the black rectangular area represents the printing head whereas the vertical length thereof represents the width of the printing made by the printing head.
However, in the case of the bidirectional printing method, in applying the reacting liquid and the ink on the printing medium overlapping with each other, the order of applying the reacting liquid and the ink during the forward scan is reverse to that during the backward scan, thereby possibly causing the occurrence of uneven coloring and resultant deterioration of the printing quality due to the bidirectional printing process.
FIG. 1A and FIG. 1B are diagrams schematically illustrating the condition described above. As shown in FIG. 1A, the arrangement of the printing heads for the inks, i.e., cyan (C), magenta (M), yellow (Y), black (K), and the reacting liquid Sp, is made so that the printing heads for the respective inks of colors are arranged along a direction of the scanning while the printing head for the reacting liquid Sp is arranged at one end of the series of printing heads for the inks. Further, in the diagram, each row of ejection orifices of the ink and the row of the reacting liquid ejection orifices are represented by the segment of straight line respectively. The same applies to the cases of other drawings referred later.
With the arrangement of the printing heads, in the case of 1-pass and bidirectional printing, for example, as shown in FIG. 1B, during the first pass of the forward scan, the overlapped application is made in the order of the reacting liquid Sp and ink M, while, during the second pass of the backward scan, the overlapped application occur in the order of the ink M and the reacting liquid Sp. In consequence, the order of the overlapped application of the ink and the reacting liquid during the forward scan differs from that during the backward scan, thereby causing the difference in the coloring between the image printed during the forward scan and the image printed during the backward scan, and then the delicate difference may be caused in the coloring of the printed image between scanning areas of the respective forward and backward scans to be unevenness coloring. Such situation is considered to result mainly from the difference in permeability to the printing medium between the reacting liquid and the ink and resultingly the amount of reacting of the reacting liquid with the ink varying depending on which of the reacting liquid and the ink is applied before the other.
In contrast, Japanese Patent Application Laid-open No.2001-138554 proposes a system wherein, as shown in FIG. 2A, the printing heads for ejecting the reacting liquid Sp are arranged symmetrically similarly to the printing heads for respective color inks (i.e., C, M and Y) so that the orders of overlapping of the ink and the reacting liquid during respective forwarding and backward scans can be made to coincide with each other. In other words, as shown in the same figure, out of the printing heads for the reacting liquid Sp, one arranged on the leftmost end and the group of the printing heads for the respective color inks arranged on the left-hand side are used together for printing during the forwarding scan, while the printing heads for the reacting liquid arranged on the rightmost end and the group of the printing heads for the respective color inks arranged on the right-hand side are used together for printing during the backward scan, whereby, as shown in FIG. 2B, it can be made possible for the reacting liquid Sp to be applied always in first during any of the forward and the backward scans, and, subsequently, any one of inks C, M, Y or two or three different color inks can be applied in the order of C, M and Y.
However, arranging the printing heads for the reacting liquid in addition to the printing heads for the respective color inks symmetrically with one another causes an increase in the number of printing heads and then causes an increase in the size of an apparatus using the printing heads and the manufacturing cost for the apparatus. Further, even if printing heads are configured so that printing heads for respective inks are recognized by a row of ejection orifices and are of chip forms which are integrated as one unit, such a system also causes an increase in the unit size and then causes an increase in the size of the apparatus. Further, the increase in the number of the printing head or the number of the chips in the fashion described above requires recovery units such as the caps, blades or the like being provided according to the printing heads, and then brings an increase in the size of the apparatus, the complication of the system of the apparatus and the increase in the manufacturing cost.
Further, the arrangements of the printing heads shown in FIG. 1A and FIG. 2A respectively are designed so that the printing heads for ejecting the ink and the printing heads for ejecting the reacting liquid are arranged on a common scanning line. Thus, such printing head arrangement is apt to give rise to a problem such that bounce mists are caused when the reacting liquid ejected and landed to a printing medium, and that the mists of the reacting liquid adheres to ejection orifice surfaces of the printing heads for inks to form insoluble substances resulting from the reaction of the reacting liquid with the ink, which provides an adverse effect on the ejection of the ink.
As the system for reducing the problem relating to the increase in the size of the printing head unit and the like, Japanese Patent Application Laid-open No.2001-138554 discloses a printing head arrangement in which the row of the reacting liquid ejection orifices is arranged to be shifted along a feeding direction of a printing medium (hereinafter referred to as a sub-scan direction) from rows of the ink ejection orifices.
FIG. 3A shows an example of such arrangement of the printing heads. In the system shown in the same figure, the respective rows of the ejection orifices for respective inks C, M and Y are arranged symmetrically with respect to the row of the ejection orifices for ink K, while the row of the ejection orifices for the reacting liquid Sp is arranged adjacent to the endmost row of the ink ejection orifices in the sub-scan direction (a sheet feeding direction). Further, the length of each row of the ink ejection orifices is set equal to the length of the row of the reacting liquid ejection orifices. According to this arrangement, as shown in FIG. 3B, in each scanning area, the reacting liquid is applied precedently by 1 pass to that inks are applied (i.e. during the 0th scan prior to the first scan for the ink; during the second scan prior to the first scan for the reacting liquid; during the third scan prior to the second scan for the reacting liquid and so on). More specifically, the inks are landed on the reacting liquid deposited during the scan preceding by 1 pass, and then the ink and the reacting liquid react with each other on the printing medium.
According to this arrangement, an order in which the reacting liquid and the ink overlap with each other can be kept constant regardless of the direction of scan as well as different scanning areas can be assigned to the reacting liquid to be ejected and the ink is to be ejected, whereby the effect of the mist of the reacting liquid can be reduced.
However, in performing printing during 1 pass by using the vertically arranged printing heads as are shown in FIG. 3A, when the difference in permeability between the ink and the reacting liquid is relatively large, such difference in the permeability may cause insufficient coloring in the vicinity of the boundary of the adjacent scanning areas, and then an printed image has white streaks throughout the whole printed image.
More specifically, in the case shown in FIG. 3B, if the permeability of the ink to be applied over the previously applied reacting liquid is higher than the permeability of such reacting liquid, the reacting liquid, which has been deposited on the printing medium preceding by 1 pass to the deposit of the ink, will be mixed to some extent with the ink, which has been deposited simultaneously with the reacting liquid during the same scan (i.e., the first scan, the second scan and whatever), in a hatched vicinity area of a boundary for the adjacent scanning area (on the right-hand side in the figure), and, as a result, the permeability of the reacting liquid mixed with the ink increases. Then, before the ink is applied in the following scans (i.e., the second scan, the third scan and whatever), the reacting liquid in the vicinity area marked with the hatching permeates a printing medium more than the reacting liquid in an area other than area marked with the hatching. In consequence, an amount of reacting of the ink with the reacting liquid in the hatched area decreases and then solubilization or coagulation of the coloring substance in the ink becomes insufficient, so that the marked area with the hatching has a lower optical density than that of the area other than the marked area. Then, the area having lower optical density can cause the problems such as the development of white streaks in the printed image.
Here, the cause of the phenomenon called the white streaks will be discussed specifically. Here, the discussion will be confined to the scanning area X, wherein the reacting liquid is applied during the first scan while the high-permeability ink is applied during the second scan (i.e., the area wherein the area 1 for application of the reacting liquid and the area 2 for application of the ink overlap with each other) and the scanning area Y, wherein the low-permeability reacting liquid is applied during the second scan while the high-permeability ink is applied during the third scan (i.e., the area wherein the area 2 for application of the reacting liquid and the area 3 for application of the high-permeability ink overlap with each other). Within the scanning area Y, the ink applied during the third scan reacts with the reacting liquid applied during the preceding second scan. In this arrangement, since the major portion (indicated as the non-hatched portion in the figure) of the scanning area Y is covered with the low-permeability reacting liquid, a sufficient amount of reacting liquid remain near the surface of the printing medium throughout the scanning area Y. Therefore, within the major portion (indicated as a non-hatched portion in the figure) of the scanning area Y, the ink and the reacting liquid can react sufficiently with each other to provide a sufficient optical density. However, the reacting liquid present within the portion indicated as the hatched portion in the figure of the scanning area Y has been mixed to some extent with the ink applied within the scanning area X during the second scan prior to application of the ink during the third scan, so that the permeability of the reacting liquid has been increased. In consequence, at the time of the third scan for application of the ink, the reacting liquid applied on the hatched area of the scanning area Y has already permeated into the printing medium to some extent. Consequently, the amount of the reacting liquid remaining near the surface of the printing medium within the hatched area (i.e., the amount of the reacting liquid for enabling the reaction with the ink to be applied during the third scan) becomes relatively small compared with the reacting liquid present within non-hatched area. In such a situation, the optical density of the hatched area becomes lower than that in the non-hatched area thereby causing the development of the white streak.